According to this Quora answer on the exact same topic:
> The primary uses, however, remain those that suggest weakness. Thus the early use to mean a girl or woman, which would become the more recent chick. Such chickens also suggest the 'cuteness' and 'fluffiness' of the creature.
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> The first example we have of chicken meaning a coward comes in 1600. It runs thus:
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>> 1600 W. Kemp Nine Days’ Wonder: It did him good to have ill words of a hoddy doddy! a hebber de hoy!, a chicken! a squib.